The technology for traditional quadrupole ion trap was greatly developed after its invention in the 1950's, and was applied in a wide variety of mass spectrometer instrument system. Many articles and patents related to this field were collected in the book “Practical Aspects of Ion Trap Mass Spectrometry” written by R. E. March and J. F. J. Todd. Usually, the three-dimensional ion trap (3D-IT) with rotary symmetry comprises a trapping volume for mass analyzing surrounded by a ring electrode and an opposing pair of end-cap electrodes. RF voltage is applied on the ring electrode to form a substantial quadrupole field to confine the ions, and a dipole AC voltage is applied between the opposing pair of end-caps to excite the ions motion and ejected out mass-selectively, to achieve mass scan of the ion trap.
The two-dimensional linear ion trap (2D-LIT) mass spectrometry instruments have been widely used because of their high sensitivity and storage capacity after commercialization. There are many designs for 2D-LIT. Commonly, as shown in FIG. 1, the 2D-LIT comprises two pairs of main electrodes 1 and 2 placed in X and Y directions perpendicular to each other, which are applied with a pair of opposite phased high frequency driving voltages separately, to form a two-dimensional linear quadrupole radial trapping electric field. Through the method of separating X and Y electrodes 1 and 2 into three segments (front 3, middle 4 and rear 5) or setting a pair of front and rear end-caps, another DC or AC axial trapping field along the trap axis (Z direction) can be formed. Usually, the ions are injected from one end along the Z axis into the ion trap and are confined in the linear shaped volume between the X and Y electrode pairs. If an additional dipole excitation voltage is applied between the X pairs of electrodes, the confined ions can be resonantly excited according to their mass-to-charge ratios and ejected out through the outlet slits 3 on X pairs of electrodes to realize mass-scan function when the amplitude or the frequency of the driven voltage is scanned.
Over years, many scientists made effort on improving the performance of ion trap in mass scan through optimizing the trapping field. For example, to overcome the effects of negative fringe field around the ejection hole during resonant ejection in 3D-IT, Kawato et al. introduced embossment flanges on the round edge of the ejection hole in U.S. Pat. No. 6,087,658. For the same problem, in U.S. Pat. No. 6,911,651, Senko et al. stretched the distance between the end-caps and made concentric recess around the outlet hole.
Above all, field improvement in ion trap by amendment on electrodes highly depends on the mechanical accuracy. Once the modified electrode is formed, the amendment of field is fixed and optimized for certain analytical condition. If the working cycle of ion trap contains more than one stage and needs different field optimization conditions, these methods may not be useful.
The designer produces a kind of 3D-IT with more than one circular electrodes in U.S. Pat. No. 5,468,958. These electrodes are applied with RF voltage of different ratios. The electric field can be adjusted by changing the ratios. An amended electrode is embedded in the end-cap electrode to introduce a field component which can be adjusted by voltages to optimize trapping field in a small range (in U.S. Pat. No. 7,279,681, L I Gangqiang et al). While in U.S. Pat. No. 6,608,303 by Amy et al., a thin metal electrode on which a RF potential with particular phase was applied, was embedded in the ejection hole to optimize field around.
The design and accuracy are simplified. The field inside can be adjusted through outside. and these technologies are used on linear ion trap gradually. In CN1585081, Chuanfan Ding designed a kind of linear ion trap surrounded by printed circuit boards. As using a lot of individual adjustable electrodes, flexible field adjustment, as well as larger ion capacity and lower cost are achieved.
But in all the above technologies, all the electrodes invoked to correct electric field depend on high frequency power supply which can accurately control voltages that are applied on these electrodes. This high frequency power supply could be a usual RF-resonant high frequency power supply or alternatively a high frequency switch power supply used by digital ion trap. Anyway, the instruments become complicated with the additional power supply.
A field adjusting electrode is placed behind the injection hole of one end cap in 3D ion trap and is driven by a DC voltage to affect respectively ion motions during injection and ejection in U.S. Pat. No. 7,285,773 by Dingli. Although this kind of local corresponding correction hasn't fully improved high frequency field components, yet as for ions which are excited, motion characteristics have been greatly improved. Since field adjusting electrode only needs to apply with a DC voltage rather than a high frequency voltage, instruments could be simplified and adjustment could be easy. But this patent is not for linear ion trap shown in FIG. 1. For linear ion trap, an ejection hole is usually made in a pair of electrodes (for example, in X direction). To ensure zero potential along axis, the pair of electrodes are applied with RF voltages or high frequency switch voltages of opposite phase with the ones applied on another pair of electrodes (in Y direction). Since there is no such zero position as in AC potential applied on end-caps in 3D ion trap, it has difficulties setting field adjusting electrode and applying voltages.
Besides, ions could eject from the two through slots on X electrodes after resonant excitation in linear ion trap, so two detectors need to place behind X electrodes to obtain maximum signal which may increase cost.